Effect of High Voltage in Transmission System

Effect of High Voltage in Transmission System

Looking at the advantages and disadvantages of the two systems, it can be concluded that high voltage transmission is advantageous. Let us study the effect of increased voltage level of transmission on

1. Volume of copper used for transmission

2. Efficiency of the line

3. The line voltage drop

 

1. Effect of High Voltage on Volume of Copper

Let a three phase a.c. system is used for the transmission. The various parameters are,

P = Power transmitted in kW

V = Line voltage in volts,       

cos ϕ = Power factor of load

l = Length of line in metres,    

A = Area of cross-section in m²

ρ = Resistivity of conductor material,

R = Resistance per conductor in Ω

The resistance per conductor is given by,

R = ρl / A ... (7.15.1)

The load current I can be obtained as,

It can be seen from the equation (7.15.5) that the volume of copper required is inversely proportional to the square of the transmission voltage and the power factor, for given P, W, ρ and l.

Thus greater is the transmission voltage level, lesser is the volume of copper required i.e. the weight of copper used for the conductors. The conductor material required is less, for higher transmission voltage.

 

2. Effect of High Voltage on Line Efficiency

The power input to the line can be written as,

P_in = P_out + Losses

Now P_out is P as considered above while the losses are given by the equation (7.15.3).

Mathematically above equation can be approximately written, using Binomial theorem as,

 η ≈ 1 - √3 ρlJ / V cos ϕ ... (7.15.11)

So for constant values of ρ, l and J, the equation (7.15.11) shows that line efficiency is higher for higher transmission voltages.

 

3. Effect of High Voltage on Line Drop

Line drop = I × R

Using equation (7.8.1),

The equation (7.15.12) shows that higher is the transmission voltage level, lesser is the percentage line drop.

 

4. Advantages of High Voltage Transmission

Electrical energy is generated at a voltage about 11 kV using alternators. This voltage is then stepped up to 132, 220 or 400 kV for transmission purpose. For transmission of electric power high voltage is preferred because of following advantages, 

1) Reduction in the current

Power transmitted is given by,          P = √ 3VI cos ϕ

where          V = Transmission voltage,  I = Load current, cos ϕ = Load power factor

Hence load current is given by,

I = P / √3V cos ϕ

From the above expression it can be seen that for the constant power and power factor, the load current is inversely proportional to the transmission voltage. With increase in transmission voltage, load current gets reduced. As current gets reduced, size of conductor required also reduces for transmitting same amount of power.

2) Reduction in the losses

Power loss in a line is given by, W = 3I² R

From the above expression it can be seen that power loss in a line is inversely proportional to square of transmission voltage i.e. greater the transmission voltage lesser is the loss in the line.

3) Reduction in volume of conductor material required.

4) Decrease in voltage drop and improvement of voltage regulation

The voltage drop in the transmission line is given by,

Voltage drop = 3IR

With reduction in current due to increase in voltage, voltage drop in the line reduces.

As J, ρ, l are constants, regulation of the line is improved when voltage increases.

5) Increase in transmission efficiency

6) Increased power handling capacity

Power transmitted over a transmission line is given by

P = V_S • V_R / X sin δ

Thus if we assume that V_S = V_R then power transmitted is proportional to square of voltage which increases power handling capacity of the line.

7) The number of circuits and the land requirement reduces as transmission voltage increases.

8) The total line cost per MW per km decreases considerably with the increase in line voltage.

9) The operation with HV A.C. voltage is simple and can be adopted easily and naturally to the synchronously operating a.c. systems.

10) The equipments used in HV A.C. system are simple and reliable without need of high technology.

11) The lines can be easily tapped and extended with simple control of power flow in the network.

 

5. Disadvantages of High Voltage

The major problems that can be occurred with HV transmission system are as follows :

1) Corona loss and radio interference

The corona loss is greatly influened by choice of transmission voltage. If weather conditions are not proper then this loss further increases. There is also interference in radio and TV which causes disturbance.

2) Line supports

In order to protect the transmission line during storms and cyclones and to make it wind resistant, extra amount of metal is required in the tower which may increase the cost.

3) Erection difficulties

There are lot of problems that arise during the erection of HV lines. It requires high standard of workmanship. The supporting structures are to be efficiently transported.

4) Insulation needs

With increase in transmission voltage, insulation required for line conductors also increases which increases its cost.

5)  The cost of transformers, switchgear equipments and protective equipments increases with increase in transmission line voltage.

6) The HV lines generates electrostatic effects which are harmful to human beings and animals. 

Review Questions

1. Explain the effect of high voltage on volume of copper and efficiency.

2. What are the advantages of high transmission voltage for the transmission of electric power ?

May-07, Dec.-08, Marks 8

3. For the same power transmitted over the same distance, show that the increase in transmission voltage of a transmission line results in : i) Increased efficiency ii) Decreased line loss iii) Reduced weight of conductor material